Apparatus and method for explosive forming



Nov. 23, 1965 w. HAWKESWORTH ETAL 3,213,834

APPARATUS AND METHOD FOR EXPLOSIVE FORMING Filed Aug. 28. 1961 INVENTORSWILLIAM HAWKESWORTH- DEXTER W. POWELL FM (.M

AGENT United States Patent 3,218,834 APPARATUS AND METHOD FOR EXPLOSIVEFORMING William Hawkesworth, Denver County, and Dexter W.

Powell, Arapahoe County, Colo, assignors to Martin- MariettaCorporation, a corporation of Maryland Filed Aug. 28, 1961, Ser. No.134,353 8 Claims. (CI. 72-56) This invention relates to apparatus andmethods for explosive forming. In particular, this invention relates toapparatus and methods for deforming a blank into the cavity of a die bysetting off an explosive charge in proximity to the blank.

In the past, the forming of parts from a single blank or sheet ofmaterial into various configurations was accomplished generally bypurely mechanical devices. One such mechanical device which has beenquite popular is the stamp press or drop hammer which employed a largemass moving in a rapid motion to drive the blank into a die. However,the mechanical devices are bulky machines which are very expensive andmust be permanently installed. Furthermore the use of these devices toproduce parts in a complex configuration from a single blank required amultiplicity of dies and/or forming operations if the part could bemechanically formed to the configuration at all.

Accordingly, industry has cast about to find other methods and apparatusfor forming parts from a blank. One forming operation that has receivedconsiderable attention from industry utilizes the blast from anexploding charge to force the blank into the die. If this forming isaccomplished by exploding the charge in the air near the blank, damageto the blank and excessive heating thereof often results. Furthermore,forming by air explosion is a relatively inefficient process unless acontainer for the explosion is provided, such container preferably beingclosed and sealed to retain the powder gases therin.

Several methods and apparatus have been suggested to alleviate the blankheating and damage problem and to increase the efficiency by explodingthe charge in a relatively incompressible material such as water. Thenby means of the incompressible material, the explosive forces aretransmitted directly to the blank surface thus accomplishing thedeforming operation. Unfortunately, the dynamic action of the shockfront transmitted by the incompressible medium and the inertial andstructural restraint of the blank combine so that the blank is deformedin a generally parabolic shape. Thus the center of the blank hits thebottom of the die first thereby trapping air between the sides of thedie and the deforming blank. This trapped air not only prevents completeforming of the part to the die configuration but reacts in an adiabaticmanner to release sufficient heat to damage the part and possibly eventhe die. Evacuation of the die cavity has partially alleviated thisproblem.

However, the most desirable manner of evacuating the die is to connect aport to the lowest point of the cavity and then to pull as efficient avacuum as possible on the port. The disadvantage of this arrangementespecially in forming dome-shaped objects is that the parabolicdeflection of the blank will bottom against the port and prevent apocket or pockets of air from escaping. The air so trapped, of course,will prevent complete forming in conformance with the die.

Accordingly, the present invention provides a method and apparatus forcontrolling the explosive deformation of a blank in a forming operation.More particularly, this invention provides a method and apparatus forcontrolling the distribution of the forces acting upon a stock blank asa result of the exploding of a charge in a relaused in accordance withthe present invention.

Patented Nov. 23, 1965 tively incompressible medium by interposing agenerally compressible gaseous material between the blank and theincompressible medium. By use of this interposed gas, the forces actingupon the blank are controlled so as to produce a relatively smoothradially inward movement of the blank thus permitting the blank toprogres sively form into the cavity of the die. For dies having anevacuation port in the lower portion of the cavity thereof, thisinvention is particularly useful in that the progressive forming willactually aid in the evacuation of any residual gases or matter from thedie cavity.

In addition, the present invention can be modified to control thepresure of the interposed gas whereby the shock front can be closelycontrolled to provide a variety of force patterns acting upon the blank.It is to be understood that the terms blank and stock-blank are usedinterchangeably herein.

The drawing illustrates an apparatus which can be A container 10 and die11 are oriented so that a blank 12 is retained therebetween. Blank 12 issimply a sheet of material which is to be formed and could be of anymaterial or in any configuration. For forming domes, it has been foundto be convenient to use a circular sheet of material for blank 12.Container 10 holds a substantially incompressible medium 14 which formany applications could simply be water. However, other liquids such asbarite or galena Weighted oil well drilling mud, heavy or morecompressible liquids, and even solids such as talc have been employedwith varying degrees of success for medium 14. An explosive charge 16 issuspended in medium 14 so as to be spatially displaced from blank 12.

In the apparatus shown in the drawing, a flexible diaphragm 17 isarranged so as to interpose a gas or relatively compressible materialbetween medium 14 and blank 12 in the area or volume indicated generallyat 18. Diaphragm 17 is sealed to a blank holder 20 which can be used tomechanically retain blank 12 against die 11. Blank holder 20 acts as abrake during the forming operation to prevent circumferential bucklingor Wrinkling due to over flow or the inertia of the moving blank. Thatis to say, buckling or wrinkling of blank 12 particularly at the edgesthereof can be reduced by employing blank holder 20.

Hole 21 through blank holder 20 is arranged so that the gas containedtherein and in space 18 will be in contact with the surface of blank 12opposite the cavity 26 of die 11. A port 22 is connected into space 18through blank holder 20. By connecting a means for supplying andwithdrawing gas to port 22, the pressure in space 18 can be controlledwhich in turn provides a means of controlling diaphragm 17 so that itcan be caused to assume a flat, concave or convex configuration beforecharge 16 is exploded. Reversible, variable delivery pump 23 and valve28 are shown as a typical arrangement for providing the means ofcontrolling the orientation of diaphragm 17.

In the drawing, die 11 is shown as having a removable upper portion ordraw-die 24 which defines the upper portion of cavity 26. The mainreason for this arrangement is so that the initial bending radius forcavity 26 can be varied simply by replacing draw-die 24 with anotherdraw-die that will mate with the lower portion of die 11. It is to beunderstood that die 11 could be made in one piece if desired and theconfiguration of cavity 26 as well as the position and/or number ofexhaust ports are dependent only upon the desired configuration of thefinished part.

As mentioned hereinbefore, it has been found to be advantageous toevacuate the cavity 26 so as to produce a substantial vacuum thereinduring the forming operation. Accordingly, cavity 26 is connected to avacuum inducing means such as pump 29 by means of port 27.

Next consider the actual accomplishment of a form ing operation inaccordance with the present invention. In particular, consider that acharge 16 is suspended in place as shown in the drawing in water formedium 14. Also space 18 has been charged with a quantity of gas viaport 22 after which port 22 is sealed by a check valve or other means sothat no gas can escape from space 18. Diaphragm 17 in some actuallyperformed tests was simply a polyethylene membrane and the gas which itretained was air although this invention is obviously not limited toeither of these specific examples.

After charge 16 is exploded, the shock-front created thereby willcompress the gas in space 18 which will start the deformation ofstock-blank 12 by a relatively uniform force distribution across thesurface thereof. Thus as stock-blank 12 begins to move into cavity 26,it will be forced against the bending radius and sides of draw die 24with approximately the same force as that causing the downward movement.The stock-blank 12 will thereby be caused to conform to the sides andthence to the bottom of cavity 26 by a relatively smooth, progressiveflowtype of deformation without prematurely sealing exhaust port 27.

It has been found that excellent results can be obtained by maintainingthe gas pressure in space 18 so that diaphragm 17 assumes the generalshape of a concave bulge into medium 14 at the time charge 16 isexploded as is shown generally in the drawing. It is believed that theconcave bulge of the gas in space 18 retracts the flat or convex shockwave advancing downward from the explosion into a more concave frontwhich reacts against stock-blanklZ. This increases the force acting atthe side of blank 12 as it deforms and improves the flow distribution inthe part.

Although obviously all that occurs after the explosion of charge 116 isnot rigorously known, some pertinent observations can be made. Forinstance, it is believed that progressive compression of the gasconverts a portion of the applied shock to potential energy for delayedand more sustained work. Further, it is believed that a slower and moreuniform acceleration of the blank is produced which lowers thedifferential particle velocity and materially reduces excessive thinningand fracture hazards. Since relatively compressible gases such as airhave .less viscosity and mass than incompressible materials such aswater, the force of the air cushion is more easily and uniformlydirected normal to the rapidly changing surface of the part duringforming. This produces lateral forming superior to that of anypreviously known explosive forming apparatus or processes. That is tosay, the separation of the medium from the blank with a resilientcushion produces an increased development and release of usefulafterflow energy from the medium entering the cushion area and the diecavity.

It should be noted that the diaphragm 17 need not be designed to stretchinto cavity 26. In fact, it has been found to be satisfactory to allowdiaphragm 17 to rupture after it has deflected towards cavity 26. Acavitation effect is thus produced as the gas in space 18 will form amultitude of bubbles which collapse from the extreme compression therebycreating cavitation forces which hammer blank 12 to the shape formingsurfaces of die 11.

There are many variations of the materials that can be used for thepresent invention. The diaphragm 17 could be polyethylene or aluminumfoil or stainless steel sheeting or copper or any of a wide variety ofmaterials as long as the amount of energy absorbed by the diaphragm iscompatible with the forming operation. In fact, diaphragm 17 could beomitted entirely and blank 12 and die 11 placed over container in aninverted arrangement from that shown in the drawing. By such anarrangement space 18 containing the gas could be controlled entirely bythe amount of incompressible medium 14 that is used. In an arrangementin accordance with the structure actually shown in the drawing, the topof container 10 could include a blow-out plate as shown or it could beleft open to the atmosphere.

There are also many types of explosives available for charge 16. Forinstance, the deflagrating or low explosive materials such as smokelesspowder or black powder could be used, these type explosives generallyhaving a burning rate along a column in the order of hundreds of feetper second dependent upon the degree of confinement. However, cautionshould be observed in utilizing the nitrocellulose-type powders becauseslightly excessive amounts of the charge can produce a highorderreaction with detonation causing bomb-like results.

Detonating or high explosives such as dynamite, TNT, RDX (primacord) orPETN are characterized by a reaction zone traveling through the chargeat velocities of thousands of feet per second. Because of the speed ofreaction, complete in only a few millionths of a second, a detonatingtype explosive releases its energy at a constant rate regardless ofconfinement. When a high explosive is detonated, a shock wave isgenerated in the containing medium with the initial velocity ofthousands of feet per second which generates an initial presure ofseveral million p.s.i. followed by a gross movement of the medium awayfrom the detonation center (afterflow). Accordingly, high explosives aregenerally preferred for explosive forming since confinement with the useof heavy, expensive clamping equipment is not required and the containercan be designed for multiple re-use. Also, there are known techniquesfor shaping a high explosive charge to control the magnitude anddirection of detonation force. Clearly these techniques could beadvantageously utilized within the spirit of this invention. Therefore,control of the explosive pressure forming distribution can be achievedby varying the mass distribution and/or shaping of the explosive chargeas well as by varying the distance between the point of explosion andthe blank (commonly referred to as, standofi).

Many variations of the method and apparatus disclosed by this inventionwill be readily apparent to one having normal skill in the art withoutdeparting from the spirit of this invention. For instance, the die 11,gasfilled space 18 and blank 12 could be immersed in a pit containingthe incompressible medium in which a charge is exploded therebyproviding a readily adaptable container for medium 14. Also space 18could be provided simply by gluing or holding a gas filled bag againstblank 12 thereby removing the need for blank holder 20. This would beespecially useful for bulging or bulgeforming of parts. If a largervolume of gas-filled space is needed, a sleeve with diaphragm 17 sealedthereto could be added on top of hole 21 in blank holder 20.

Those having normal skill in the art will appreciate the fact that thereare a multitude of arrangements available for retaining the componentsshown in the drawing in proper position. By way of example, cables andclamps could be used to secure container 10 to blank holder 20 althoughit would probably be desirable in so doing to leave some pressurerelieving arrangement such as the blow out plate in the top of container10. Further, through-bolts or clamps could be employed to retain die 11and blank holder 20 in position. However, it should be appreciated thatthe apparatus of the drawing could be successfully utilized as shownsince the vacuum drawn on cavity 26 would ensure that blank 12 and theparts of die 11 would remain in position and the weight of container 10with medium 14 contained therein could be sutficient to maintain contactwith holder 20 and diaphragm 17 for a sufiicient length of time to allowthe forming operation to be performed.

What we claim is:

1 An improved apparatus for explosively forming parts from a stock-blankcomprising:

a preformed die having a cavity therein;

an explosive charge;

a substantially incompresible medium containing said charge therein;

a blank holder for forcibly retaining said blank against said die andhaving a port therethrough arranged for introducing a gas to the surfaceof said blank opposite the cavity of said die;

a flexible diaphragm sealably attached over said blank holder port anddefining with said holder and blank a container for retaining apressurized gas;

means for retaining said incompressible medium in contact with saidflexible diaphragm; and

means for maintaining the cavity of said die substan tially at a vacuumduring the deforming operation, said medium transmitting the forcesgenerated by the explosion of said charge so as to deflect saiddiaphragm towards said blank thereby compressing the gas and causingforces substantially normal to the surface of said blank throughout thedeformation thereof into the cavity of said die.

2. An improved apparatus in accordance with claim 1 which includes meansfor selectably controlling the pressure of the gas retained between saidblank and said medium.

3. An improved apparatus in accordance with claim 1 t which includes acontainer in fixed relation to said blank holder for retaining saidmedium and concentrating towards said diaphragm the forces generated bythe explosion of said charge.

4. An improved method of explosively deforming a blank into the cavityof a preformed die comprising the steps of:

retaining said blank against the die;

interposing a confined gas between a substantially incompressiblematerial and the surface of said blank opposite the cavity of said die,said gas being at a pressure sufiicient to displace a portion of saidincompressible material;

controlling the pressure of said confined gas to present a substantiallyconcave bulge into said substantially incompressible material; and

exploding an explosive charge in said substantially incompressiblematerial.

5. An apparatus for explosively forming parts from a stock-blankcomprising:

a die having a cavity for forming said stock-blank;

means for holding a stock-blank against said die;

means for generating an explosiv force;

means for containing a substantially incompressible medium on the sideof said stock-blank opposite said die, said incompressible medium beingadapted to transmit the forces created by said explosive forcegenerating means toward said stock-blank; and

means for supplying gas under pressure between said stock blank and saidincompressible means for transmitting the explosive forces into forcesacting substantially normally to the surface of said stock-blank duringdeformation thereof into said cavity of said die.

6. An apparatus for explosively forming parts from a stock-blank, as setforth in claim 5 further including:

a flexible diaphragm between said containing means and said gassupplying means, said diaphragm being adapted to be supported by thepressure of said gas and extending concavely into said incompressiblemedium.

7. An apparatus for explosively forming parts from a stock-blank, as setforth in claim 5, further including:

means for evacuating the cavity of said die.

8. An improved apparatus in accordance with claim 5 which includes meansfor selectably controlling the pressure of said gas between saidstock-blank and said incompressible means.

References Cited by the Examiner UNITED STATES PATENTS 2,935,038 5/ 1960Chatten 113-44 3,044,430 7/ 1962 Zeigler 11344 3,068,822 12/1962 Orr etal 113--44 FOREIGN PATENTS 766,741 1/ 1957 Great Britain.

CHARLES W. LANHAM, Primary Examiner.

NEDWIN BERGER, Examiner.

1. AN IMPROVED APPARATUS FOR EXPLOSIVELY FORMING PARTS FROM ASTOCK-BLANK COMPRISING: A PERFORMED DIE HAVING A CAVITY THEREIN: ANEXPLOSIVE CHARGE; A SUBSTANTIALLY INCOMPRESSIBLE MEDIUM CONTAINING SAIDCHARGE THEREIN; A BLANK HOLDER FOR FORCIBLY RETAINING SAID BLANK AGAINSTSAID DIE AND HAVING A PORT THERETHROUGH ARRANGED FOR INTRODUCING A GASTO THE SURFACE OF SAID BLANK OPPOSITE THE CAVITY OF SAID DIE; A FLEXIBLEDIAPHRAGM SEALABLY ATTACHED OVER SAID BLANK HOLDER PORT AND DEFININGWITH SAID HOLDER AND BLANK A CONTAINER FOR RETAINING A PRESSURIZED GAS;MEANS FOR RETAINING SAID INCOMPRESSIBLE MEDIUM IN CONTACT WITH SAIDFLEXIBLE DIAPHRAGM; AND MEANS FOR MAINTAINING THE CAVITY OF SAID DIESUBSTANTIALLY AT A VACUUM DURING THE DEFORMING OPERATION, SAID MEDIUMTRANSMITTING THE FORCES GENERATED BY THE EXPLOSION OF SAID CHARGE SO ASTO DEFLECT SAID DIAPHRAGM TOWARDS SAID BLANK THEREBY COMPRESSING THE GASAND CAUSING FORCES SUBSTANTIALLY NORMAL TO THE SURFACES OF SAID BLANKTHROUGHOUT THE DEFORMATION THEREOF INTO THE CAVITY OF SAID DIE.